Continuous evaporator

ABSTRACT

A continuous evaporator for a horizontal waste heat steam generator is provided. The continuous evaporator includes a first evaporator heating surface that has a number of essentially vertically arranged first steam generating tubes wherein the circulation takes place from the bottom to the top, and another second evaporator heating surface located downstream of the first evaporator heating surface on the side of the flow medium, the second evaporator heating surface including a number of other essentially vertically arranged second steam generating tubes wherein the circulation takes place from the bottom to the top is provided. In order to provide a continuous evaporator with an especially simple structure and an especially long service life, a number of second steam generator tubes have an inner profiled element.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is the U.S. National Stage of International Application No. PCT/EP2010/055886, filed Apr. 30, 2010 and claims the benefit thereof. The International Application claims the benefits of Getman application No. 10 20009 024 587.1 DE filed Jun. 10, 2009. All of the applications are incorporated by reference herein in their entirety.

FIELD OF INVENTION

The invention relates to a once-through evaporator for a horizontal-type waste heat steam generator, having a first evaporator heating surface comprising a plurality of essentially vertically disposed first steam generator tubes through which a flow medium flows from the bottom to the top, and another second evaporator heating surface disposed downstream of the first evaporator heating surface in respect of the flow medium direction and comprising a plurality of other essentially vertically disposed second steam generator tubes through which the medium flows from the bottom to the top.

BACKGROUND OF INVENTION

In a gas and steam turbine plant, the heat contained in the expanded working medium or heating gas from the gas turbine is used to generate steam for the steam turbine. Heat transfer takes place in a waste heat steam generator mounted downstream of the gas turbine and in which a plurality of heating surfaces for water preheating, steam generation and steam superheating are normally disposed. The heating surfaces are connected into the water/steam circuit of the steam turbine. The water/steam circuit nonually contains a plurality of, e.g. three, pressure stages, each pressure stage possibly having an evaporator heating surface.

For the steam generator mounted downstream of the gas turbine in the heating gas path as a waste-heat steam generator, a number of alternative design concepts are suitable, namely configuration as a once-through steam generator or as a recirculating steam generator. In the case of a once-through steam generator, the heating of steam generator tubes provided as evaporator tubes results in evaporation of the flow medium in the steam generator tubes in a single pass. In contrast, in the case of a natural or forced circulation steam generator, the circulating water is only partly evaporated as it passes through the evaporator tubes, the water that is not evaporated being is re-fed to the same evaporator tubes for further evaporation after separation of the generated steam.

In contrast to a natural or forced circulation steam generator, a once-through steam generator is not subject to any pressure limitation. A high live steam pressure promotes high thermal efficiency and therefore low CO₂ emissions from a fossil-fired power plant. In addition, a once-through steam generator has a simple type of construction compared to a recirculating steam generator and can therefore be manufactured particularly inexpensively. Using a steam generator of once-through design as the waste heat steam generator of a gas and steam turbine plant is therefore particularly advantageous for achieving a high overall efficiency of the gas and steam turbine plant without constructional complexity.

A once-through steam generator designed as a waste heat steam generator can basically be implemented in one of two alternative types of construction, namely vertical or horizontal. A horizontal-type once-through steam generator is designed to provide an approximately horizontal flow path for the heating medium or heating gas, e.g. the exhaust gas from the gas turbine, whereas a vertical-type once-through steam generator is designed to provide an approximately vertical flow path for the heating medium.

In contrast to a once-through steam generator of vertical design, a horizontal-type once-through steam generator can be manufactured using particularly simple means and with particularly low production and assembly costs. In both types, particularly in the downstream (in flow medium direction) steam generator tubes of the second evaporator heating surface, an uneven distribution of the two-phase flow medium to the steam generator tubes may occur within each individual row of tubes, resulting in temperature imbalances and mechanical stresses caused by differential thermal expansion. In order to prevent damage to the waste heat steam generator, expansion bends, for example, have therefore hitherto been fitted to compensate for these stresses. However, this measure can be comparatively complex technically in the case of a horizontal-type waste heat steam generator.

SUMMARY OF INVENTION

The object of the invention is therefore to specify a once-through evaporator for a waste heat steam generator as described in the introduction, which permits a particularly simple type of construction while providing a particularly long service life.

This object is achieved according to the invention by providing a plurality of second steam generator tubes with an internal profile.

The invention is based on the consideration that a particularly simple design of the waste heat steam generator or more specifically of the once-through evaporator could be achieved by dispensing with the hitherto customary expansion bends. In doing so, however, another way must be found of reducing the mechanical stresses caused by temperature imbalances in the parallel connected steam generator tubes of each individual tube row. These occur particularly in the second evaporator heating surface to which the water/steam mixture is applied. The temperature imbalances are caused by different proportions of water and steam in the flow medium at the inlet of the individual tubes of a tube row and a resultant differential flow through these tubes. It was realized that this differential flow in the tubes is caused by the frictional pressure drop in the steam generator tubes being low compared to the geodetic pressure drop. In fact, a flow with a high proportion of steam in the flow medium flows comparatively quickly through individual steam generator tubes with low frictional pressure drop, whereas a flow with a high proportion of water is disadvantaged because of its higher geodetic pressure drop due to weight and may tend to stagnation. In order to make the flows uniform, the frictional pressure drop must therefore be increased. This can be achieved by a plurality of second steam generator tubes having an internal profile which causes an additional frictional pressure drop of this kind.

In order to a achieve a particularly high additional frictional pressure drop, the laminar boundary layer on the inside of the tubes must be reduced. This can be achieved by producing turbulence in the tube. This effect can be amplified still further by swirling of the flow medium. Swirling of this kind can be produced by advantageously making the internal profile helical-spring-shaped.

Said frictional pressure drop must be appropriately determined using the normal operating parameters such as tube geometry, the dimensions of the heating gas path and the temperature conditions. Advantageously, the geometry of the particular internal profile must then be selected such that the predefined frictional pressure drop of the flow medium is obtained via the respective second steam generator tube. This provides an even better means of preventing temperature imbalances.

In an advantageous embodiment, the particular internal profile is provided in the respective second steam generator tube as a kind of internal finning, thereby allowing a particularly simple once-through evaporator or rather waste heat steam generator design.

In order to facilitate retrofitting to existing steam generators and/or achieve greater flexibility in the design of steam generators in respect of the tube geometries, the particular internal profile is advantageously inserted in the respective second steam generator tube as a fitted component. The internal profile is therefore implemented as a separate component and disposed in the steam generator tubes.

In an advantageous embodiment, a plurality of second steam generator tubes are connected in series in the heating gas direction as tube rows. This enables a larger number of parallel connected steam generator tubes to be used for an evaporator heating surface, which means a better heat input due to the enlarged surface. However, the steam generator tubes disposed in series in the heating gas flow direction will be differentially heated in this case. The flow medium will be comparatively strongly heated particularly in the steam generator tubes on the heating gas inlet side. However, as a result of the described design of the second steam generator tubes with an internal profile, a through-flow matched to the heating can also be achieved in these steam generator tubes, thereby ensuring a particularly long service life for a waste heat steam generator of simple constructional design.

In an advantageous embodiment, the first evaporator heating surface is connected downstream of the second evaporator heating surface in respect of the heating gas direction. The advantage of this is that the second evaporator heating surface connected downstream in respect of the flow medium direction and therefore designed for further heating of already evaporated flow medium is also in a comparatively more strongly heated region of the heating gas path.

A once-through evaporator of this kind can be usefully installed in a waste heat steam generator and the waste heat steam generator employed in a gas and steam turbine plant. Said steam generator is advantageously connected downstream of a gas turbine in respect of the heating gas direction. In this configuration, supplementary firing equipment can be usefully connected downstream of the gas turbine to increase the heating gas temperature.

The particular advantages achieved by the invention consist in that, by providing the second evaporator tubes with an internal profile, an improvement is achieved in the distribution of the flow and therefore a reduction in the temperature differences between parallel connected second steam generator tubes and in the resultant mechanical stresses, thereby ensuring a particularly long service life of the waste heat steam generator. The appropriate provision of the internal profile enables other complex technical measures such as expansion bends to be dispensed with, while at the same time allowing a particularly simple, cost-saving design of the waste heat steam generator or more particularly of a gas and steam turbine power plant.

BRIEF DESCRIPTION OF THE DRAWINGS

An exemplary embodiment of the invention will now be explained in greater detail with reference to the accompanying drawings in which:

FIG. 1 shows a simplified view in longitudinal section through a horizontal-type steam generator,

FIG. 2 shows a longitudinal section through a steam generator tube with internal finning,

FIG. 3 shows a longitudinal section through a steam generator tube with fitted components,

FIG. 4 shows a plot of tube temperature versus steam content at the heating tube inlet without internal profile and

FIG. 5 shows a plot of tube temperature versus steam content at the heating tube inlet with internal profile

Identical parts are denoted by the same reference characters in all the figures.

DETAILED DESCRIPTION OF INVENTION

The once-through evaporator 1 for the waste heat steam generator 2 as shown in FIG. 1 is connected downstream of a gas turbine (not shown in greater detail) in respect of the exhaust gas direction. The waste heat steam generator 2 has an enclosing wall 3 which forms a heating gas path 5 for the exhaust gas from the gas turbine which can flow through it in an approximately horizontal heating gas direction indicated by the arrow 4. A plurality of evaporator heating surfaces 8, 10 of once-through design are disposed in the heating gas path 5. Although in the exemplary embodiment in FIG. 1 two evaporator heating surfaces 8, 10 are shown, a larger number of evaporator heating surfaces can also be provided.

The evaporator heating surfaces 8, 10 shown in FIG. 1 each comprise, in the manner of a tube bundle, a plurality of tube rows 11 and 12 respectively, disposed in series in the heating gas direction. Each row of tubes 11, 12 in turn comprises a plurality of steam generator tubes 13 and 14 respectively, disposed in parallel with one another in the heating gas direction, only one of which is visible for each tube row 11, 12. Said approximately vertically disposed first steam generator tubes 13 of the first evaporator heating surface 8 which are connected in parallel for the passage of a flow medium W are connected on the output side to a common outlet header 15. The likewise approximately vertically disposed second steam generator tubes 14 of the second evaporator heating surface 10 which are connected in parallel for the passage of a flow medium W are likewise connected on the output side to a common outlet header 16. A comparatively more complex header system can also be provided for the two evaporator heating surfaces 8, 10. A downcomer system 17 connects the steam generator tubes 13 of the first evaporator heating surface 8 to the downstream (in terms of flow medium) steam generator tubes 14 of the second evaporator heating surface 10.

The flow medium W which can be applied to the evaporator system constituted by the evaporator heating surfaces 8, 10 is evaporated in one passage through the evaporator system and, after exiting the second evaporator heating surface 10, is discharged as steam D. The evaporator system formed by the evaporator heating surfaces 8, 10 is connected into the water/steam circuit (not shown in greater detail) of a steam turbine. In addition to the evaporator system comprising the evaporator heating surfaces 8, 10, a number of other heating surfaces 20 schematically indicated in FIG. 1 can be connected into the water/steam circuit of the steam turbine. Said heating surfaces 20 can be, for example, superheaters, medium pressure evaporators, low pressure evaporators and/or preheaters.

The second steam generator tubes 14 now have a helical-spring-shaped internal profile 22 which is illustrated in FIGS. 2 and 3. The profile geometry thereof is selected such that the swirl- and turbulence-induced frictional pressure drop of the flow medium W in the steam generator tubes 14 is accordingly high enough to ensure an even through-flow within a tube row 11, thereby reducing temperature imbalances. Said internal profile 22 can be directly incorporated in the steam generator tubes 14 as a kind of internal finning 23. Alternatively, fitted components 24 can be used as the internal profile 22, which in particular allows retrofitting to existing once-through evaporators 1.

The effect of the internal profile 22 on the temperature differences is illustrated in the graphs in FIGS. 4 and 5 which plot the average tube wall temperature 25 and the tube outlet wall temperature 27 against the steam component 29 of the flow medium at the tube inlet. FIG. 4 shows the situation without internal profile 22. Here the average tube wall temperature 25 varies between approximately 460° C. and 360° C., the tube outlet wall temperature 27 between 480° C. and 370° C., as a function of the steam content 29. In FIG. 5 which explains the situation with internal profile 22 it is shown that these variations are reduced to approximately 440° C. to 390° C. and 470° C. to 405° C. respectively. The temperature differences between tubes with different steam content at the inlet are therefore significantly reduced.

By reducing the temperature differences of tubes with differing steam content at the flow inlet, the mechanical stress loading of the waste heat steam generator 2 is reduced and a particularly long service life is ensured, while at the same time allowing a simple type of construction by eliminating the hitherto customary expansion bends. 

1-9. (canceled)
 10. A once-through evaporator for a horizontal-type waste heat steam generator, comprising: a first evaporator heating surface comprising a first plurality of essentially vertically disposed first steam generator tubes through which a flow medium flows from bottom to top; and a second evaporator heating surface connected downstream of the first evaporator heating surface with respect to the flow medium direction and comprising a second plurality of essentially vertically disposed second steam generator tubes through which the flow medium flows from bottom to top, wherein the second plurality of second steam generator tubes each include an internal profile.
 11. The once-through evaporator as claimed in claim 10, wherein the internal profile is helical-spring-shaped.
 12. The once-through evaporator as claimed in claim 10, wherein the profile geometry of the respective internal profile is selected such that a specified frictional pressure drop of the flow medium is obtained via a respective second steam generator tube.
 13. The once-through evaporator as claimed in claim 10, wherein the respective internal profile is provided as a kind of internal finning in a respective second steam generator tube.
 14. The once-through evaporator as claimed in claim 10, wherein the respective internal profile is installed as a fitted component in a respective second steam generator tube.
 15. The once-through evaporator as claimed in claim 10, wherein the plurality of second steam generator tubes is connected in series as tube rows in a heating gas direction.
 16. The once-through evaporator as claimed in claim 10, wherein the first evaporator heating surface is connected downstream of the second evaporator heating surface with respect to the heating gas direction.
 17. A waste heat steam generator, comprising: a once-through evaporator as claimed in claim
 10. 18. The waste heat steam generator as claimed in claim 17, wherein a gas turbine is disposed upstream of the waste heat steam generator with respect to a heating gas direction.
 19. The waste heat steam generator as claimed in claim 17, wherein the internal profile is helical-spring-shaped.
 20. The waste heat steam generator as claimed in claim 17, wherein the profile geometry of the respective internal profile is selected such that a specified frictional pressure drop of the flow medium is obtained via a respective second steam generator tube.
 21. The waste heat steam generator as claimed in claim 17, wherein the respective internal profile is provided as a kind of internal finning in a respective second steam generator tube.
 22. The waste heat steam generator as claimed in claim 17, wherein the respective internal profile is installed as a fitted component in a respective second steam generator tube.
 23. The waste heat steam generator as claimed in claim 17, wherein the plurality of second steam generator tubes is connected in series as tube rows in a heating gas direction.
 24. The waste heat steam generator as claimed in claim 17, wherein the first evaporator heating surface is connected downstream of the second evaporator heating surface with respect to the heating gas direction. 